1. Field of the Invention
The invention relates to an optical system for providing a useful light beam influenced by polarization. The invention relates, more particularly, to optical systems which provide a useful light beam for wafer exposure in microlithographic projection exposure machines.
2. Description of the Related Art
In the named specific application of wafer exposure, one or more deflecting mirrors are frequently used in the optical system, which is typically composed of an illuminating system upstream of the reticle plane or mask plane, and a projection objective between the reticle plane and wafer plane. As is known, p-polarized light, that is to say light polarized linearly parallel to the plane of incidence, is attenuated more strongly in the case of reflection at a deflecting mirror than is s-polarized light, that is to say light polarized linearly perpendicular to the plane of incidence. On the other hand, it is desirable in conjunction with high requirements placed on the imaging quality that the p-component and the s-component of the illuminating light are as equal as possible in the wafer plane.
This problem is addressed, for example, in the Laid-Open Patent Application DE 198 51 749 A1 for the case of a catadioptric objective with a plurality of deflecting mirrors, the latter also to be understood in the present case as appropriate deflecting prisms. It is proposed there as a remedy to arrange the deflecting mirrors in a compensated fashion independently of polarization, specifically with planes of incidence which are not parallel, and, in particular, perpendicular to one another. This remedial measure is, however, suitable only for systems having a plurality of deflecting mirrors, and the design of the illuminating system and projection objective in the overall optical system does not always permit such an arrangement of deflecting mirrors with non-parallel planes of incidence. The measure proposed there as an alternative, of providing the at least one deflecting mirror of an objective with a thin, polarization-specific layer, in particular a phase-correcting dielectric layer, can be implemented only with difficulty for applications with large beam divergences and short wavelengths.
U.S. Pat. No. 5,475,491 discloses an exposure system in which a fraction of the light generated by a laser, for example, is coupled out to a photodetector at a semi-transmitting mirror by means of reflection, while the light fraction transmitted by the semi-transmitting mirror forms the useful light beam for exposure. The exact exposure dose is to be detected by the detector. Since, however, the respective degree of reflection and transmission of the semi-transmitting mirror differs for the s-polarized component and the p-polarized component, the detector result cannot be used directly to infer the true exposure dose when the polarization state of the incoming light is not known or varies upstream of the deflecting mirror. As a remedy, a plane transmission plate is introduced obliquely relative to the optical axis with an adjustable angle of inclination into the beam path of the coupled-out detection light between the deflecting mirror and detector. This angle of inclination is then set, as a function of the degree of transmission and reflection of the semi-transmitting mirror, for s- or p-polarized light such that the light intensity arriving at the detector is always proportional, independently of the possibly varying s-polarized or p-polarized light fractions, to the intensity of the useful light transmitted by the semi-transmitting mirror. The relative fractions for the s- and p-components of the useful light are not fixed thereby and can vary as desired depending on the design of the system part upstream of the semi-transmitting mirror.